fix(completion): offer unique hostnames without instance prefix

bash (and zsh without a matcher) only accept candidates that start with
the typed text, so "fsn1-bx1<Tab>" found nothing when every candidate
was qualified as "prod/…". Hostnames that exist in only one instance
are now offered bare; names present in several instances stay
qualified. Connecting to a bare name still checks every instance for
ambiguity, so the connection behavior is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/netssh/instances_test.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"reflect"
 	"strings"
 	"testing"
 	"time"
@@ -206,3 +207,33 @@ func TestRefreshCachesIndependentDevicesAndVMs(t *testing.T) {
 		}
 	}
 }
+
+func TestHostCompletionsMatchTypedText(t *testing.T) {
+	entries := []cache.Entry{
+		{Name: "prod/fsn1-bx1.example.com"},
+		{Name: "prod/fsn1-bx2.example.com"},
+		{Name: "stg/fsn1-bx2.example.com"},
+		{Name: "stg/hel1-tb1.example.com"},
+		{Name: "prod/HEL1-TB2"},
+	}
+	for _, tc := range []struct {
+		input string
+		want  []string
+	}{
+		// Unique names are bare; names in both instances stay qualified.
+		{"fsn1", []string{"fsn1-bx1.example.com", "prod/fsn1-bx2.example.com", "stg/fsn1-bx2.example.com"}},
+		{"hel1", []string{"HEL1-TB2", "hel1-tb1.example.com"}},
+		{"prod/", []string{"prod/HEL1-TB2", "prod/fsn1-bx1.example.com", "prod/fsn1-bx2.example.com"}},
+		{"stg/fsn", []string{"stg/fsn1-bx2.example.com"}},
+		// An instance name typed without the slash completes qualified targets.
+		{"st", []string{"stg/fsn1-bx2.example.com", "stg/hel1-tb1.example.com"}},
+	} {
+		if got := hostCompletions(entries, tc.input); !reflect.DeepEqual(got, tc.want) {
+			t.Errorf("%q: got %v, want %v", tc.input, got, tc.want)
+		}
+	}
+	// Single-instance caches are unchanged.
+	if got := hostCompletions([]cache.Entry{{Name: "web01"}, {Name: "db01"}}, "w"); !reflect.DeepEqual(got, []string{"web01"}) {
+		t.Errorf("single instance: %v", got)
+	}
+}